Ad-hoc wireless networks offer communications solutions where individual devices (or nodes) can communicate directly with other nodes, without any network provisioning or central points of failure. For example, in an ad hoc wireless network, two wireless radios can communicate directly with each other, or through other radios on the network, instead of having to transmit through a dedicated central access point. These networks traditionally use carrier sense multiple access/collision avoidance (“CSMA/CA”) as their media access control mechanism, as they have no central control entity. A media access control, or MAC, mechanism provides a protocol for accessing and controlling communication channels. However, CSMA/CA does not perform well in the presence of a highly-varied channel or over large distances. The CSMA/CA protocol is built on the assumption that all nodes can detect the transmissions of the other nodes that they may attempt to communicate with.
A simple string of pearls configuration (several nodes in a linear configuration) can easily create a situation where two nodes, at both ends of the string, attempt to communicate with a node in the middle of the string and, because the two transmitting nodes cannot hear each other, both end nodes transmit messages to the middle node. The result is that the middle node hears both transmissions, but cannot decode either transmission. This is known as a “collision.” This simple example is known as the “hidden node” problem for CSMA/CA. A mechanism generally accepted in the art to address this is the use of a two-way handshake using a Request to Send (“RTS”) message and Clear To Send (“CTS”) message with network allocation vectors in each message. The use of RTS/CTS in a network, while addressing the hidden node problem, substantially reduces the capacity of the network. Moreover, it does not address many of the other scenarios under which CSMA/CA will perform sub-optimally.
As a CSMA/CA network can be operated with larger distances between nodes, the protocol needs to be altered to provide sufficient gaps in transmissions to ensure that there is no overlap. The IEEE 802.11g standard, used for example, to create an in-home wireless computer network, and an example of a CSMA/CA protocol, calls for 9.6 μsec between the end of one discrete time slot and the beginning of the next slot. The concept of time slots allows several nodes to transmit on the same frequency channel by transmitting at their own times, or time slots. This 9.6 μsec time was chosen to support the propagation delay between nodes, the spread of the waveform over a multi-path environment (where transmissions can follow any of a number of paths from one node to another), the computational time to decode the information transmitted in a slot, and timing margin to simplify the implementation. As the network is extended from, for example a 100 meter separation between nodes to a 5 km separation to a 50 km separation, the round-trip propagation time grows from 66 nsec, to 3.2 μsec, to 32 μsec. In order for a CSMA/CA protocol to operate properly over these distances, the size of the gaps between transmissions needs to be extended, also reducing the efficiency of the protocol.
Various approaches have been researched to scale the capacity of wireless networks. The CSMA/CA protocol essentially divides the total capacity (C) among the nodes, so the per-node capacity at node i, c(i), shrinks as the number of nodes, N, increases, according to the equation c(i)=C/N. In other words, since the total capacity remains the same, as the number of nodes increases, each individual node's capacity decreases. This is driven by a fundamental assumption that CSMA/CA is designed upon that all nodes must take turns sharing the available capacity.
The capacity (C) of a network can be spatially divided into non-overlapping regions. To achieve a number k independent regions among the physical area covered by the network, the capacity per node c(i) would increase by a factor of k, according to the equation c(i)=k*C/N. However, such a media access control mechanism does not work well for a highly-dynamic collection of nodes. Beam-forming antennas, which can transmit signals directionally, can be used for creating these k independent regions. Although beam-forming could provide for better separation of regions, it exacerbates the problem that CSMA/CA has since it does not have a uniform channel between all the nodes in a neighborhood. Protocols that use a form of time-division multiple access (“TDMA”) mechanism and pass control of the channel from one node to another work poorly when faced with a “fast-mover” node (e.g., a cell phone in a car on the expressway) that traverse many neighborhoods rapidly. Using beam-forming approaches also generally requires adding the concept of state to the protocol, which requires more management exchanges and lengthening of the time for the neighborhood to adjust to any changes in the population. This additional overhead reduces capacity and efficiency.
Aspects and embodiments disclosed herein are directed to addressing/solving these and other needs.